A device and method for optically measuring a substance in blood using the nail as an optical window are provided. A device for optically measuring a test substance in blood by using the nail as an optical window and correcting or eliminating fluctuation of a measurement value based on optical characteristics of the nail plate, comprising irradiation means applying light within a wavelength range for measuring absorption by the test substance to the nail of a subject, detection means detecting light diffused/reflected from or transmitted through the body of the subject, and processing means processing a signal obtained by the detection means to convert the signal into a concentration of the test substance.